1. Field of the Invention
The present invention relates to superplastic forming of multiple sheets in a manifold-free system, and more particularly to forming or shaping metal sheets with internally generated gas pressures.
2. Description of Related Art
Superplastic forming technology (SPF) has been frequently used in the aerospace industry to manufacture near net shape and stress free articles (i.e., components) through low strain rate forming operations under an applied pressure at elevated temperatures. Applied pressures and elevated temperatures have produced elongations up to 8000% or more in metals, up to 800% or more in intermetallics, up to 1400% or more in metallic composites, up to 1025% or more in ceramics, and up to 625% or more in ceramic composites. Recent SPF techniques involve laser welding or diffusion bonding to seal and join two or more sheets together in strategic locations so that when the assembly is pressurized with an inert gas at elevated temperature, the sheets inflate to fill the inside of a sealed die. After cooling, the manufactured component takes on the shape of the die, and may contain integrally stiffened members that are created when the strategically placed welds or bonds act as pinning points in the forming operation. Such multiple sheet SPF technology may show great promise at manufacturing complex shape structural components for the aerospace and other industries and has some advantages over conventional wrought metal forming processes and the like.
However, the commercial application of welded and SPF components has been economically limited, particularly due to high capital costs of SPF presses, to low throughput through the presses (e.g., batch modes) and by restrictions caused by connecting pre-inflated components to high pressure gas manifolds. Long forming times, on the order of hours, have discouraged improved SPF efforts.
In recent years, internally generated gas pressures have been employed to inflate malleable metal sheets. Trenkler et al. in U.S. Pat. No. 4,434,930 describe painting and sealing a pattern of thermally decomposable stop-off material onto an interfacial surface of two or more metal sheets, then solid phase green bonding the sheets and raising the temperature to decompose the stop-off material, thus generating gas and inflating the sheets contiguous to the pattern. However, such a technique suffers from employing inadequate and otherwise uncontrollable amounts of stop-off materials and consequently generating inadequate gas pressures. Oftentimes, the once-sealed painted patterns of the techniques such as those of Trenkler and others fail to provide additional stop-off material that can be added to generate additional internal gas pressure. Furthermore, the methods employing the once-sealed painted patterns have difficulty regulating the strain rate of inflatable superplastic materials and the like.
Accordingly, a need exists for more economical SPF methods that avoid batch processes and promote conveyor belt type processes, can avoid having to attach the components or articles to a high pressure manifold and can be manifold free, and offer flexibility in controlling or regulating fluid pressures during the formation of such articles.